1. Field of the Invention
The invention generally relates to a transistor, and more particularly, to a thin film transistor (TFT).
2. Description of Related Art
A TFT can be roughly categorized into a staggered one and a coplanar one based on the structure layout of the layers thereof. For the staggered TFT, the semiconductor layer is disposed between the gate layer (or the insulation layer) and the source/drain layer thereof; for the coplanar TFT, the source/drain layer is disposed the gate layer (or the insulation layer) and the semiconductor layer thereof. In addition, the coplanar TFT can be further divided into a top gate type where the gate layer is located at the upper portion of the TFT and a bottom gate type where the gate layer is located at the lower portion of the TFT.
For the current coplanar TFT, the carrier transmission channel over the source/drain layer of the TFT is affected by a shielding effect of the source/drain layer where the electrical field produced by the gate layer below the source/drain layer is shielded, so that there is no charge accumulation at the region where the electrical field is shielded, which leads to increasing the impedance of the carrier transmission channel and reduces the mobility of carriers thereof.